A wide variety of equipment and systems, such as portable and desktop computers, mainframe computers, communication infrastructure frames, automotive equipment, etc., include heat-generating components in their casings. As increasingly dense and higher performance electronics are packaged into smaller housings, the need for effective cooling systems is paramount to prevent failure of such sensitive electronics devices. One method used to remove heat from such equipment is to have an axial fan draw air from the exterior of the casing to blow cooling air over the heat-generating components. However, as the number of electronics devices in offices and households increase, so too does the number of cooling fans. As such, fan noise becomes significantly loud and undesirable.
Noise reduction in fans generally is accomplished through either active and/or passive noise reduction techniques. In a passive noise reduction system, a fan may include a plurality of projections having a number of predetermined masses that are arranged at positions around the periphery of the blade. This results in creating an unstable mode for the fan. The unstable mode results in disruption of airflow over the blade, thereby resulting in less noise at the trailing edge. However, such a system requires the fan to rotate at a preset rotational speed for maximum effectiveness. Rotation of the fan at other than the preset speed results in decreased effectiveness of the noise reduction methods.
An active noise reduction method includes a fan having a micro electro mechanical system that includes a thin silicon film forming an integrated circuit and an actuator connected to the circuit for generating vibrations. The fan reduces noise by causing the actuator to generate vibration that offsets or reduces unstable airflow along the blade body. However, the operation of the noise reduction system is less than optimal because the actuator and the sensing portion are configured as a closely spaced, or even single, device that is placed at one particular portion of the fan blade. Thus, the actuator and the sensing portion are separated by a negligible distance. As such, the system is unable to simultaneously sense the wake at the trailing edge of the blade and create turbulent flow at a predetermined point along the fan blade.